The capability for routine and accurate characterization—and correction—of geometric distortion is becoming increasingly important for MRI applications in image-guided therapy. Applications where accurate geometrical measurements from MRI images are required include image-guided radiotherapy (Crijns 2012; Aubry 2010; Chen 2006), quantitative brain imaging (Maikusa 2013), and quantification during imaging for osteoarthritis (Schneider 2013) and the preparation of patient-specific positioning guides (Krishnan 2012). In order to correct for inherent geometric distortion, a variety of fiducial grids and sheets have been proposed, typically based on regularly structured 3D grids (Baldwin 2007; Wang 2004a; Wang 2004b; Kiryu 2011; Mizowaki 2000; Stanescu 2010; Stanescu 2012), rods (Doran 2005; Tanner 2000), or 3D distributions of glass marker beads (Viard 2008). Grid phantoms based on commercially fabricated polystyrene grids suffer from manufacturing imprecision and difficulty in post-processing and analysis to determine line intersections. Glass marker beads placed in custom-fabricated trays are complicated to fabricate.
Thus, there is still a need for dimensionally accurate 3D grid phantoms that are able to be imaged using MRI or CT techniques, are readily processable using automated techniques to determine grid intersections, and are simple to manufacture.